The process for the development of hematopoietic cells from a self-regenerating stem cell to the mature multi-lineage cells has been the subject of intense investigation. How the host is able to direct a single cell into the multiplicity of pathways which provide such varied cells as lymphocytes, monocytes, macrophages, megakaryocytes and osteoclasts, which is not a complete list, still remains to be elucidated. However, substantial strides have been made in identifying various intermediate cells associated with the different lineages and identifying factors which appear to direct, either by themselves or in combination with other factors, the cells to mature to a particular cell type. For the most part, the factors have been associated with progenitors which are committed to a particular lineage, where the factors result in maturation of the progenitor. Less is known about the factors which direct a totipotent or multipotent cell to be directed to one among many possible lineages.
In order to understand the processes of differentiation and maturation, it will be necessary to know which surface membrane proteins act as receptors for transduction of signals, whether they have soluble forms, the role of soluble forms, and which ligands are associated with the surface membrane proteins. It is therefore of substantial interest to be able to identify ligands, receptors and their soluble forms associated with cell regeneration, proliferation, differentiation, and maturation.